This invention relates to increasing the storage density of optical drives, for example, magneto-optical, compact disk, and write-once-read-many (WORM) drives. For purposes of illustration, this specification focuses on magneto-optical drives. It will be apparent to those skilled in the art that the principles of the invention can be readily applied to other optical drives as well.
In the writing process of typical magneto-optical drives, a laser produces a light beam that passes through an objective lens that focuses the light beam onto a magneto-optical disk. The central portion of the light beam heats a microscopic spot on a magnetic layer within the magneto-optical disk, while a biased magnetic field is applied in the vicinity of the light spot on the disk. The heat of the light spot temporarily lowers the coercivity of the magnetic layer. In the central, hottest portion of the light spot the coercivity of the magnetic layer is lowered enough that the magnetic field causes the magnetic orientation of the magnetic layer to reverse direction. As the magnetic layer cools, the local orientation of the magnetic layer becomes fixed to form a domain on the disk representing a bit of information. The presence of a domain having a reversed magnetic orientation at a given location on the disk represents a "1" while the absence of such a domain represents a "0."
In the reading process of typical magneto-optical drives, a laser produces a polarized light beam that passes through a first polarized beam splitter and passes through an objective lens that focuses the light beam onto a magneto-optical disk. The information stored at the point on the magneto-optical disk at which the light beam is focused causes the polarization of the light beam to shift slightly clockwise or slightly counterclockwise, depending on whether the information stored on the disk is a "1" or a "0," as the light beam reflects off of the disk. The reflected light beam passes back through the objective lens along the same path as the path of the incident light beam. The objective lens collimates the reflected light beam, which returns to a light detector assembly.
One approach to increasing recording density, described in U.S. patent application Ser. No. 07/373,939, filed Jun. 29, 1989, by Neville Lee et al., titled "Method for Increasing Track Density of Magneto-Optical Storage Media," and assigned to the assignee of the instant invention, involves constructing guard bands on the disk between concentric tracks on the disk, or between turns of a spiral track on the disk. With this arrangement, the number of concentric tracks (or the number of turns of a spiral track) can be increased, because the guard band can be located under an upper portion of the light spot as the drive reads one track (or turn of a spiral track), and under a lower portion of the light spot as the drive reads a neighboring track (or turn of a spiral track), without interference occurring between domains of information in neighboring tracks (or neighboring turns of the spiral track). In other words, the light beam is not affected by the domains of more than one track (or turn of a spiral track) at a time.
Another approach to increasing recording density, disclosed in U.S. patent application Ser. No. 07/373,991, now U.S. Pat. No. 5,161,134, filed Jun. 29, 1989, by NeVille Lee, titled "Method for Increasing Linear Bit Density in Magneto-Optical Storage Media," and assigned to the assignee of the instant invention, involves locating a slit between the objective lens and the disk. The length of the slit is parallel to the radial direction on the disk. The lengthwise edges of the slit cut off portions of the light beam, thereby allowing the domains of information on a given track to be recorded closer to each other without interference occurring between adjacent domains on a given track. In other words, the light beam is not affected by more than one domain within a track at a time.